The present invention relates to novel maleimide and alicylic olefin-based monomers, a copolymer resin of these monomers, and a photoresist using the copolymer resin. More specifically, the present invention relates to a maleimide- or alicyclic olefin-based monomer useful for a photoresist usable for lithography process using ultra-short wavelength light source such as KrF(.lambda.=248 .mu.m), ArF(.lambda.=193 .mu.m), X-ray, ion beam, E-beam and EUV(Extreme Ultra Violet) which is a potentially applicable technology to the fabrication of a fine circuit of a highly integrated semiconductor device. The present invention also relates to a copolymer resin of these monomers and to a photoresist formulated by using the copolymer resin.
Recently, attempts to highly integrate a semiconductor device have had a big influence on the development of fabrication techniques for photoresist patterns. To prepare a semiconductor device, it is necessary to formulate a photoresist pattern, which is widely used in masking such as etching or ion injection.
The general mechanism of formulating a photoresist pattern is as follows. First, polymer and photoacid generator are dissolved in solvent in a certain ratio to give a photoresist solution. This photoresist solution is coated on a wafer substrate of semiconductor device, and then selectively exposed through a mask to a light source. The exposed wafer is soft-baked to remove an exposed or un-exposed region of the photoresist by using a weak alkaline developing solution, such as tetramethyl ammonium hydroxide (TMAH). The soft-baked wafer is washed with deionized water and dried to formulate a photoresist pattern.
The relation between photoresist resolution and wavelength of light source in photolithography is shown by the following formula: EQU R=k*.lambda./NA
wherein R is a resolution, k is a process constant, .lambda. is wavelength of a light source, and NA is a numerical aperture. As shown in the above formula, the resolution (R) of a photoresist pattern is proportional to the wavelength (.lambda.) and the process constant (k), and is in inversely proportional to the numerical aperture (NA) of laser stepper. Thus, to increase the light resolution, the wavelength of light source must be shortened. For instance, G-line and I-line laser steppers which are 436 nm and 365 nm in wavelength are limited to about 0.7 .mu.m and 0.5 .mu.m in the process resolution, respectively.
In attempts to formulate a fine pattern of 0.5 .mu.m or less, a stepper using a deep ultra violet (DUV) light source of a short wavelength, such as KrF laser (.lambda.=248 nm) or ArF laser (.lambda.=193 nm), a contrast enhancement layer (CEL) process by which a separate thin film capable of improving image contrast is formulated on the wafer, a process for using a phase reversal mask, or a silylation process for silylating a surface of photoresist film have been proposed. However, these processes are complicated and yield is lowered. Therefore, a laser stepper using DUV light source has been developed as the most simple process. In this regard, a photoresist for DUV light source has also been developed.
As a photoresist for DUV light source, chemical amplification photoresists have been the prevailing choice in semiconductor devices, because they are highly sensitive to DUV light, which is now recognized as light source suitable for accomplishing the high integration of semiconductor devices. The chemical amplification photoresist consists generally of a photoacid generator and a matrix polymer having a chemical structure which sensitively reacts with acid.
The mechanism of such a chemical amplification photoresist is as follows. The photoresist is exposed through a mask to a ultra violet light source. An acid is generated by the action of the photoacid generator which then, reacts with the main or side chain of the matrix polymer. This reaction surprisingly increases the solubility of the copolymer in the developing solution by changing the structure of the polymer, e.g., by decomposing it, cross-linking it or changing its polarity. Therefore, at exposed regions the copolymer is dissolved in the developing solution, whereas at un-exposed regions the copolymer has no change in its original structure and remains undissolved in the developing solution, so that the shape of the mask may leave as a positive image on a substrate. In the above lithographic process, since the resolution depends on the wavelength of light source, the smaller the wavelength of light source, the finer the pattern formulated.
In general, a photoresist is required to have high etch resistance, thermal resistance and adhesion. In addition, the photoresist film used in semiconductor devices must be developed in 2.38% TMAH aqueous solution. However, it would be difficult to prepare a copolymer resin which satisfies all the properties of photoresist. For instance, the copolymer resin having polyacrylate-based main chain structure can be easily synthesized, but it is difficult to get etching resistance and to use 2.38% TMAH in development process.
The etching resistance can be improved by introducing alicyclic unit into main chain of the copolymer resin, but it is difficult to substitute the entire main chain by alicyclic unit. When a metal is used as a catalyst, the entire main chain may be substituted by alicyclic moiety. However, in this instance, removal of the metal component contained in the resin is difficult and the metal component may give a fatal unfavorable effect on the semiconductor device.
In the other hand, Bell Lab. has proposed a copolymer resin of the following formula (1), having a main chain structure substituted by norbornene, acrylate and maleic anhydride. ##STR1##
In this copolymer resin, maleic anhydride, A-moiety of the formula (1), used for the polymerization of an alicyclic olefin group does not absorb a light of 193 nm in wavelength and is the only material capable of polymerizing with the alicyclic unit, i.e., norbornene. However, the maleic anhydride, upon non-exposure, is easily dissolved in 2.38% TMAH aqueous solution and occurs top-loss phenomenon in which the top of the photoresist pattern becomes round. Therefore, this copolymer resin is not suitable to the resin for KrF or ArF lithography process.
Accordingly, to inhibit dissolution, it is necessary to increase the ratio of t-butyl substituent, y-moiety of the formula (1). In this instance, the ratio of carboxylate substituent, z-moiety of the formula (1), which increases the adhesion is reduced and thus, the photoresist is departed from the wafer upon actual patterning and the pattern formulation is not possible.
In addition, upon post exposure delay which does not involve a baking immediately after exposure, the bottom of the pattern is shorter than the top of the pattern. That is, T-top phenomenon occurs, and thus the pattern formulation itself is not possible. Also, since maleic anhydride reacts with hydroxyl groups (--OH) which increase adhesion, there is a possibility of influencing shelf life of the photoresist.
Bell Lab. has attempted to solve such disadvantages by introducing as a bi-component dissolution inhibitor, an alternating copolymer of cycloolefin and maleic anhydride. However, since this method must use a dissolution inhibitor in the excess amount, about 30% by weight of the copolymer resin, reproductivity of the resin is low and the cost is increased. Accordingly, this resin also is not suitable as a photoresist resin.